The present invention relates to signal processing and in particular to the conversion of frequency modulation (FM) signals to corresponding digital signals and/or the conversion of analogue signals to corresponding digital signals.
The present inventors have described in a paper entitled xe2x80x9cDelta-Sigma Modulators using Frequency-Modulated Intermediate Valuesxe2x80x9d (Hxc3x8vin, M., Olsen A., Lande, T. S., Toumazou, L., I.E.E.E. J. Solid State Circuits, Vol. 32, No. 1, January 1997) a circuit wherein a carrier signal is frequency modulated with an analogue input signal x(t) to produce a frequency modulated signal fm(t). The rising edges of the frequency modulated signal fm(t) are counted by a digital modulo 2n counter and the total count is sampled at a sampling frequency fs. The sampled counts are differentiated to produce a digital output yn corresponding to the analogue input signal x(t).
This known circuit exhibits good signal to noise properties as the counting operation effectively integrates the analogue input signal x(t) before the sampling operation. Thus, it is the integrated input signal that is sampled and to which quantization noise Qn is added. After the sampling operation, the quantized integrated input signal, together with the quantization noise Q, is differentiated to produce the output signal yn. The digital output signal yn is therefore given by equation 1
yn=xn+(1xe2x88x92Zxe2x88x921)Qxe2x80x83xe2x80x83(1)
where Zxe2x88x921 represents the unit delay operator and xn is the quantized value of x(t). The multiplication of the quantization noise Q by (1xe2x88x92Zxe2x88x921) is equivalent to first order delta-sigma noise shaping which results in an improvement in the signal to noise ratio of the analogue to digital converter.
This type of basic frequency to digital converter is known as a xe2x80x98count and dumpxe2x80x99 converter, as the number of rising edges of the FM signal is continuously counted and the difference between the count in successive sampling periods Ts (=1/fs) is xe2x80x98dumpedxe2x80x99 to the output. Such a count and dump system may also be used in an oversampling configuration such that the count of rising edges in successive sampling periods Ts is summed and decimated to give an estimate of the instantaneous frequency in a decimation period Td=1/fd, where fs greater than fd.
The present invention seeks to improve on the known xe2x80x98count and dumpxe2x80x99 frequency to digital converters and analogue to digital converters.
According to a first aspect of the present invention, there is provided a method of converting an oscillating signal to a digital signal representative of the frequency of the oscillating signal over a sampling interval T, wherein the occurrence of corresponding points of successive cycles of the oscillating signal are counted, the count is sampled at a first sampling frequency fs, where 1/fs less than T, to give a plurality of values of the count for a plurality of sampling sub-intervals Ts and the sub-interval counts are summed to provide an output, characterised in that each sampled count value is weighted by a weight which is a function of the position of the respective sub-interval Ts in the interval T.
Thus, according to the method of the invention weighting information is applied to the cycle count for each sub-interval Ts such that the digital output produced by this method is a weighted sum of the counts for each sub-interval Ts over the sampling interval T. In this way, information is included in the output which relates to the position of each sub-interval in the sampling interval and the signal to noise ratio is thereby increased. In known xe2x80x98count and dumpxe2x80x99 converters, the count for each sub-interval Ts has an equal weight and thus no positional information is included.
Preferably, the weighting function has a minimum value at the start and end of the sampling interval T and may have a maximum value in the centre of the sampling interval. In this way, the sub-intervals in the central region of the interval T are given a greater weight than those at the edges which may be clipped by the sampling operation. The weighting function is preferably symmetrical so that sub-intervals at equivalent portions at either side of the centre of the sampling interval are given equal weights.
In the preferred embodiment, the weighting is triangular, although other weighting functions could be used equally well. Across the interval T, the applied weight increases linearly, for example by 1 per Ts, from a minimum at the first sub-interval until the maximum weight is reached at the centre sub-interval, the weight then decreasing linearly, for example by 1 per Ts, to a minimum at the last sub-interval.
Preferably the output is provided at a frequency greater than 1/T such that the sampling intervals T associated with successive digital outputs overlap, in the sense that at least two successive digital outputs contain count values associated with, at least some, common sub-intervals Ts. In the preferred embodiment the output frequency fd (=1/Td) and T=2Td. Thus two successive digital outputs D (=Td/Ts) sub-intervals in common, although for each output the count for each Ts will be given a different weight, as it will appear at a different position in the respective sampling interval T. The quantity D is the decimation ratio.
The weighting may be applied to each count by any suitable arrangement of signal processing components. However, in the preferred embodiment the continuous count is summed over successive sub-intervals Ts, the summed count is sampled at a regular decimation interval Td and the sampled sum is differentiated twice by differentiators synchronised to Td. The output according to this method is a triangularly weighted sum of the number of occurrences of the corresponding points in successive cycles of the oscillating signal in each sub-interval Ts, across a sampling interval of 2Td.
This is in itself believed to be a novel configuration and thus in a preferred arrangement there is provided a method of converting an oscillating signal to a digital signal representative of the instantaneous frequency of the oscillating signal, wherein the oscillating signal is applied to a digital counter which counts the occurrence of corresponding points of successive cycles of the oscillating signal, the output of the counter is sampled at a first sampling frequency fs, the sampled output is summed over successive sampling sub-intervals Ts, the summed counts are sampled at a second sampling frequency fd and are then differentiated twice by successive differentiators clocked at the second sampling frequency fd to provide an output. One advantage of this method is that the quantization noise resulting from the sampling is delta sigma noise shaped.
The corresponding points on the successive cycles of the oscillating signal are preferably rising or falling edges of the signal. Advantageously the digital counter may be in the form of an accumulator clocked by the rising or falling edge of the oscillating signal and arranged to increase the stored count by one when clocked by the rising (or falling) edge of the signal. Alternatively, a latch may be used to sample a limited oscillating signal. The sampling frequency may be applied to the clocking input of the latch and the limited oscillating signal may be applied to the latch input of the latch. The output of the latch may then be used as a clocking signal for a counter.
For the above described methods to be applied to the conversion of an analogue signal to a corresponding digital signal, the analogue signal need only be used to frequency modulate a carrier frequency. The above-described methods may then be used to obtain a digital signal corresponding to the instantaneous frequency of the frequency modulated signal which will correspond to the analogue signal.
The invention also extends to the circuits employed to put the above described methods into effect.
Thus, from a second aspect the invention provides a converter for converting an oscillating signal to a digital signal representative of the frequency of the oscillating signal over a sampling interval T, the converter comprising a counter arranged to continuously arranged to sample the output of the counter at a sampling frequency fs, characterised in that the converter further comprises weighting means to produce an output in the form of a weighted sum of the sampled counts over a sampling period T.